.father {
  margin: 0 auto;
  margin-top: 50px;
  width: 300px;
  height: 300px;
  background-color: yellow;
}
.son {
  width: 200px;
  height: 200px;
  background-color: blue;
}
.grandson {
  width: 100%;
  height: 100px;
  background-color: pink;
  position: fixed;
}
